This invention relates to a color image forming device for forming toner images on a belt type image forming unit by the electrophotographic method and transferring them on a transfer material to form images.
Various methods and devices for forming color images by the electrophotographic method have been proposed. As indicated in Japanese Patent Publication-Open to Public Inspection No. 100770 (1986), for example, there is a method that latent images are formed and developed on a photosensitive drum which is an image forming unit according to the number of separated colors of an original image, and the developed images are transferred onto a transfer drum at each development to form a multi-color image on the transfer drum, and then the multi-color image is transferred onto a recording paper to obtain a color copy. A device using this method requires a transfer drum having a peripheral surface which is large enough for one image to be transferred on besides the photosensitive drum. Therefore, such a device is inevitably large in size and complicated.
As indicated in Japanese Patent Publication-Open to Publication Inspection No. 149972 (1986), for example, there is another method that latent images are formed and developed on a photosensitive drum according to the number of separated colors of an original image, and the developed images are transferred onto a transfer material at each development to form a multi-color copy. By this method, it is difficult to accurately superimpose multi-color images and good quality color copies cannot be obtained. There is another method available that the forming of latent images on a photosensitive drum according to the number of separated colors of an original image and the development of images by color toners are repeated, and the color toner images are superimposed on the photosensitive drum and transferred onto a transfer material to obtain a color image. The basic process of this multi-color image forming is indicated in Japanese patent Publication-Open to Publication Inspection No. 75850 (1985), 76766 (1985), 95456 (1985), 95458 (1985), and 158475 (1985) by the applicant of this patent.
In a multi-color image forming device for obtaining color images by superimposition, a plurality of developing units containing different colors from each other are installed around the photosensitive drum and latent images on the photosensitive drum are developed by rotating the photosensitive drum several times to obtain a color image.
As to an image forming unit, as described above, a photosensitive drum which is coated or deposited with a photoconductor on the peripheral surface thereof and a belt type image forming unit with a flexible belt coated or deposited with a photoconductor have been proposed. Since the belt type image forming unit (hereinafter may be called the photosensitive belt) is formed in a shape by stretching between rotating rollers including drive roller, it is effective for a color image forming device which is made compact for effective space use. Since the photosensitive belt can move along a small curvature, a transfer material can be separated at the curvature by using a rotating roller with a small diameter to prevent poor separation of the transfer material.
Since the belt type image forming unit mentioned above is a photosensitive thin sheet with a conductive layer inserted between the base and an organic photosensitive layer and used in an endless form, it can be manufactured as follows: A cylindrical photoconductor is formed by casting, and cut into pieces with a necessary width to produce belt type photoconductor. However, there is a problem imposed by this method. Since the thickness of each photoconductor is extremely thin such as 0.1 mm and the thickness should be accurate, the forming is technically hard and it is rather difficult to obtain a belt type image forming unit of satisfactory quality.
A method that both ends of a photoconductor sheet produced using a film (extruded and elongated thin film) of polyethylene terephthalate or others are spliced to form an endless photoconductor belt has also been tried. However, the difference in level caused by splicing both ends cannot be eliminated, no image can be formed on the spliced part, the photoconductor may slip by touching of the cleaning member, or the spliced part may be separated in an extreme case, causing problems in the durability. The first object of the present invention is to solve the above problem and to provide a color image forming device which can be used for a long period of time with a good conveyability and durability not being affected by an elastic cleaning member which acts on a belt type image forming unit which is formed by bonding.
Since the difference in level which is equivalent to the photoconductor thickness occurs at the spliced part, an unnecessary amount of toner of the developer is adhered to the step even in the non-contact development. In the case of color images, therefore, a color toner may be mixed into another color developing unit, and when a carrier is adhered, the photoconductor may be damaged during cleaning. The second object of the present invention is to solve the above problem and to provide an image forming device which can produce images of high quality without the adhierence of unnecessary toner or carrier even when a photosensitive belt having spliced part is used.